Vinyl acetate ethylene (VAE) copolymer latices have found considerable application as bases for paints and other surface coatings, as adhesives, textile treating agents, and the like. A large body of technology has developed over the years both in respect to processes for preparing these commercially important latices and to their end uses.
In general, a VAE copolymer latex is prepared by first charging an aqueous phase containing water, surfactant, buffer, catalyst or catalyst system of the free radical type, and usually a protective colloid such as polyvinyl alcohol (PVA), to a reactor. In some procedures, an initial charge of vinyl acetate monomer, and in others, the entire amount of vinyl acetate monomer, is also charged to the reactor. The reactor is flushed with nitrogen, sealed and stirring is commenced. Ethylene is then pumped to the reactor until the desired pressure is attained. The reactor can be repressurized one or more times if the batch is carried out under variable ethylene pressure, or a constant pressure can be maintained automatically employing techniques which are well known in the art. After reactor pressure has stabilized, the contents thereof are heated to the polymerization temperature, usually by circulating hot water or steam through a jacket surrounding the reactor. When the desired polymerization temperature (commonly from about 120.degree. to about 165.degree. F.) is reached, temperature is maintained at this level by automated controls. Thereafter, a co-catalyst such as sodium hydrogen sulfite (NaHSO.sub.3) can be added to the reactor (if a catalyst system employing a reducing agent to generate free radicals by a redox reaction is used) followed by any remaining vinyl acetate monomer. The completion of polymerization is indicated by cessation of ethylene demand and stabilization of the reactant coolant temperature at about 6.degree.-8.degree. F. above the reactor temperature. Upon completion of polymerization, the reactor contents are cooled and discharged through a pressure let-down valve to a storage tank at atmospheric pressure from which unreacted ethylene is vented. The finished VAE copolymer latex is passed through a screen of desired mesh to complete the manufacturing process.
Of the various tests available for measuring the physical properties/performance characteristics of VAE copolymer latices, two of the most significant are the Time of Set Test and the Vinyl Wetting Test. The Time of Set Test is a measurement of the relative adhesive set time of a VAE copolymer latex on a kraft paper substrate. This test is highly significant for packaging applications since the rate of set of the adhesive determines the operating speed of the packaging line. As is readily appreciated, a faster time of set is directly translatable to improved packaging economics. The Vinyl Wetting Test measures the relative wettability of flexible vinyl film by a VAE copolymer latex adhesive. This property is significant in relation to the commercially important application of latex adhesives for lamination of various substrates to flexible vinyl film.
Inherent viscosity is a very important physical property of VAE copolymer latices and has been found to directly influence the performance of the latices in yet another significant performance test, the Creep Test. This test measures the resistance to delamination of an adhesive bond under an applied load at elevated temperature. In general, the higher the inherent viscosity of a VAE copolymer latex, the better its performance will be in the Creep Test.
Various manipulations of both the amount and nature of the components of a VAE copolymerization medium and the copolymerization process variables have heretofore been attempted in order to optimize one or a few properties of the resulting latex. U.S. Pat. No. 3,644,262 describes a copolymerization which by regulating the addition of vinyl acetate to an aqueous emulsifying composition containing a free-radical initiator at a rate which will maintain the concentration of unpolymerized vinyl acetate at a level not exceeding about 3.5% by weight of emulsifying composition and, optionally by delaying the addition of surface active agent, permits the introduction of substantially more ethylene into the copolymer for a given pressure and temperature than would be otherwise attainable. The resulting high ethylene content VAE copolymer latices are said to be better adapted to their end uses than the latices of relatively low ethylene content. A different approach to improved VAE copolymer latices is described in U.S. Pat. No. 3,423,352 in which high solids content VAE copolymer latices of reduced viscosity and improved freeze-thaw stability are obtained by controlling the addition of monomer, catalyst and surfactant. According to this patent, relatively large amounts of surfactant, i.e., from about 3% to about 10% by weight, and catalyst are added to a conventionally prepared polyvinyl acetate latex, having a solids content of up to about 52% and containing relatively large amounts of vinyl acetate, at specified times once polymerization has proceeded to a certain extent. This is said to result in a marked reduction in the viscosity of the emulsion. Frequently, these and other prior art techniques for preparing VAE copolymer latices achieve an improvement in one or two performance characteristics but at the expense of one or more other vital performance characteristics. Thus, for example, superior performance in the Vinyl Wetting Test may be attained but at a substantial or even total loss of performance in the Creep Test.